Excess voltage arresters

ABSTRACT

An excess voltage arrester comprising a pair of spaced electrodes housed in a gas-filled enclosure wherein the electrodes carry sleeves of electrically insulating material to prevent reduction of the d.c. ignition voltage of the arrester due to the formation of conductive coatings on the internal surfaces of insulating portions of the envelope by sputtering from the discharge occurring in the arrester in operation.

United States Patent [1 1 Baker et al.

[ 1 EXCESS VOLTAGE ARRESTERS [75] Inventors: Basil Offor Baker,Richmansworth;

John William Ellis Ross, Sidcup, both of England [73] Assignee: The M-OValve Company Limited, London. England [22] Filed: Mar. 21, 1974 [2]]Appl. No.: 453,322

[30] Foreign Application Priority Data Mar. 23, 1973 United Kingdom[4138/73 [52] US. Cl 317/61; 313/231.1 [51] Int. Cl. H02h 9/04 [58]Field of Search 317/61, 61.5, 62, 66, 68; 313/214, 220, 231.1 5 6]References Cited UNITED STATES PATENTS 3,289,027 11/1966 Jones 317/61 X[11} 3,885,203 51 May 20, 1975 Primary Examiner-G. Harris Attorney,Agent, or FirmKirschstein, Kirschstein, Ottinger & Frank [57] ABSTRACTAn excess voltage arrester comprising a pair of spaced electrodes housedin a gas-filled enclosure wherein the 5 Claims, 1 Drawing FigurePATENTEB MAY 2 0 I975 EXCESS VOLTAGE ARRESTERS This invention relates toexcess voltage arresters.

A known form of excess voltage arrester comprises: a pair of spacedelectrodes housed in a hollow cylindrical, gas-filled enclosurecomprising: electrically conductive end portions and an electricallyconductive tubular central portion which is sealed to and spaced fromthe end portions by respective tubular electrically insulating portions,the end portions constituting re spective terminals for the twoelectrodes, the two electrodes extending towards each other through theinsulating portions so that the gap between the electrodes lies withinthe central portion. Such an excess voltage arrester is hereinafterreferred to as an arrester of the kind specified. One such arrester isdescribed in UK. Patent Specification No. 1,021,782.

In normal use of an arrester of the kind specified the two electrodesare respectively connected via their terminals to two lines, and thecentral portion of the enclosure is grounded. When an excess voltageappears between either one of the lines and the other line, or ground, adischarge occurs in the gas-filled enclosure, thus effectively groundingthe lines.

In an arrester of the kind specified a discharge is intended to occurbetween the inner end of one or both the electrodes and the centralportion of the enclosure, and the gap between these elements is chosenin conjunction with the pressure and composition of the gas filling toconfer a desired operating voltage on the arrester.

It has been found that with known arresters of the kind specified, aftera large number of discharges have occurred, subsequent discharges tendto occur at an intermediate point along the length of one or bothelectrodes, at a dc. ignition voltage lower than the designed voltage.

It is an object of the present invention to provide an arrester of thekind specified wherein this difficulty is overcome.

According to the present invention in an arrester of the kind specifiedeach electrode carries a sleeve of electrically insulating materialwhich extends from a point near the inner end of that electrode to apoint within the insulating portion of the enclosure through which thatelectrode extends.

In a preferred arrangement in accordance with the invention the outerend of each said sleeve lies adjacent a section of the associatedelectrode which is a sliding fit in the associated insulating portion ofthe enclosure.

The invention resides in the appreciation by the inventors that thefailure of prior art arresters results from the formation by sputteringfrom the discharge of conductive coatings on the internal surfaces ofthe insulating portions of the enclosure. The formation of such films isenhanced by a tendency for the cathode spot of the discharge occurringin operation to run along the surface of the electrode on which it isformed towards the end of the arrester.

In an arrester in accordance with the invention the sleeves reduce thelikelihood of the formation of conductive coatings on the internalsurfaces of the insulating portions of the enclosure.

In a particular arrangement in accordance with the invention each saidsleeve is effective to increase the breakdown voltage between any pointon the surface of the electrode carrying that sleeve and a conductivecoating which may be produced in operation on the internal surface ofthe adjacent insulating portion of the enclosure to a value above the dcignition voltage across the gap between the inner end of that electrodeand the adjacent part of the central portion of the enclosure.

One arrangement in accordance with the invention will now be described,by way of example, with refer ence to the accompanying drawing which isa sectional view of a gas-filled excess voltage arrester. Referring tothe drawing, the arrester includes a hermetically sealed, hollowcylindrical, gas-filled enclosure comprising two cup-shaped metal endcaps 1 and 2, a tubular metal central member 3 and two tubular ceramicmembers 4 and 5. The ceramic members 4 and 5 are sealed at one end intothe end caps 1 and 2, locating on chamfers 6, and at the other end intoopposite ends of the central enclosure member 3. To this end, the outersurface of each of the ceramic members 4 and 5 is metallised in twobands 8 and 9, annular seals being established between the inter-fittingparts by brazing during assembly.

Within the gas-filled enclosure are housed a pair of axially spacedelectrodes 10 and 11. The electrode 10 is formed in two parts 12 and 13which are brazed together, end on. The outer part 12 lies mainly withinthe ceramic envelope member 4 and comprises three sec tions 12a, 12b and12c. The central major section 12!) is a sliding fit in the ceramicmember 4. The relatively short outer section 12a is of reduced externaldiameter and is brazed to the end cap 1. The inner section 12c is ofsimilarly reduced external diameter, has a length about half that of thecentral section 12b, and at its inner end is brazed to the other part 13of electrode 10. The part 13 also comprises three sections 13a, 13b and13c, the section 130 adjacent section 12c of part 12 being of smallerdiameter than section and of relatively short length. The central majorsection 1312 is of the same external diameter as section 12c and has alength such that the inner section 130, which is of relatively largeexternal diameter, lies near the centre of the metal member 3 of theenclosure. The parts 12 and 13 have communicating axial bores 12d and13d, the bore 12d being enlarged at its outer end to accommodate apumping stem 23 which is pinched-off in conventional manner after therequired gas-filling has been inserted in the enclosure.

The other electrode 11 is brazed to the end cap 2 at one end and extendsthrough the ceramic member 5 so that, its other end lies near to, butspaced from, the corresponding end of the electrode 10, the electrode 11being otherwise identical with the electrode 10 except that it is notprovided with a pumping stem.

In the drawing the two parts of the electrode 11 are given referencenumerals 14 and 15 with suffixes a, b and c, as for electrode 10.

The gap 16 between the electrodes 10 and ll is made to have the samed.c. ignition voltage as the gaps l7 and 18 between the centralenclosure member 3 and the innermost sections 13c and 15c of theelectrodes.

A ceramic sleeve 19 fits closely around sections 120 and 13b ofelectrode 10, with its outer end abutting section 12b, and acorresponding sleeve 20 fits closely around sections 14c and 15b ofelectrode 11.

In manufacture of the arrester each end cap 1 or 2 is first brazed toits associated electrode 10 or II (with ceramic sleeve 19 or 20 inposition) and to ceramic member 4 or S to form a sub-assembly, thebrazing being carried out with the electrode axis vertical and the endcap lowermost, and with rings of a suitable brazing material placedaround the sections 12a and 13a of electrode 10 and the correspondingsections of electrode 11. The two sub-assemblies are then sealed intoopposite ends of the central member 3 of the en' closure. and therequired gas filling is inserted via the pumping stem 23.

In use of the arrester, the end caps 1 and 2 are re spectively connectedto a pair of lines connected with the equipment which it is desired toprotect against excess voltage. and the metal enclosure member 3 isgrounded. On the occurrence of an excess voltage be tween either one ofthe lines and the other line, or ground, a discharge occurs between oneor both electrodes 10 and 11 and the metal member 3 of the enclosure. Itwill be appreciated that while initially the discharge may occur betweenthe two electrodes 10 and 11 the discharge will rapidly transfer to thegap between one or both electrodes and the grounded member 3. A desiredd.c. ignition voltage for the arrester, that is the voltage at which adischarge is initiated, is obtained by appropriate choice of thepressure and composition of the gas-filling and the sizes of the gapsl6, l7 and 18.

When a discharge takes place in the arrester, sputtered material fromthe discharge forms electrically conductive coatings on the internalsurfaces of the metal member 3 and the ceramic members 4 and 5 of theenclosure. The formation of these coatings is enhanced due to a tendencyof the cathode spot of the discharge to run along the surface of theelectrode on which it is formed towards the nearer end cap 1 or 2. Itwill be appreciated that the abovementioned coatings are electricallyconnected to ground in operation via the metal member 3. Hence, ifcoatings are formed which extend along the inner curved surfaces of theceramic members 4 and 5 towards the electrode sections 12b and 14b, theoperating voltage of the arrester will eventually be very significantlyreduced, and ultimately the arrester will short circuit the lines withwhich it is connected.

In accordance with the invention, the sleeves l9 and 20 substantiallyrestrict the formation of such conductive coatings to the end faces 21and 22 of the ceramic members 4 and 5. This is achieved partly by virtueof the gap between the sleeves 19 and 20 and the ceramic members 4 and 5being too small to allow easy access of the sputtered material to theinternal curved surfaces of members 4 and 5, and partly by virtue of thesleeves 19 and 20 preventing the cathode spot of the discharge frommoving very far towards an end cap 1 or 2 in operation.

In addition, the presence of the sleeves 19 and 20 makes the shortestgap (via the gas filling) between a conducting coating on an end face 21or 22 and the adjacent electrode 10 or 11 larger than the gaps l7 and18. Thus, the presence of the sleeves 19 and 20 increases the breakdownvoltage between a conducting coating on an end face 21 or 22 and theadjacent electrode 10 or 11 to a value above the d.c. ignition voltageset by the gaps 17 and 18, thus preventing the coating reducing the dcignition voltage of the arrester.

ln one particular arrester of the form shown in the drawing having a dc.ignition voltage of 400 volts, the gas filling consists of a 10%hydrogen argon mixture at a pressure of 50 torr, the radial spacingbetween the external surfaces of the electrodes sections 12c 13b, 14cand 15b and the internal surfaces of ceramic members 4 and 5 is 0.95millimetres and the sleeves 19 and 20 have internal and externaldiameters of 2.92 millimetres and 3.98 millimetres respectively andconsist of 98% alumina.

In the arrester shown in the drawing the adjacent sections 13c and 15cof the electrodes 10 and 11 are of relatively large external diameter,but this need not be the case in an arrangement in accordance with theinvention. Thus in alternative arrangements the portions of theelectrodes projecting inwardly beyond the sleeves l9 and 20 may be ofthe same diameter as the portions within the sleeves.

It is pointed out that while the particular arrester shown in thedrawing is designed for connection in an external circuit by means ofclips or the like contacting the end caps 1 and 2 and the enclosuremetal member 3, other arresters in accordance with the invention may beprovided with lead wires for external connection.

We claim:

1. An excess voltage arrester comprising: a hollow cylindrical gasfilled enclosure comprising a tubular electrically conductive centralportion, a pair of electrically conductive end portions, and a pair oftubular electrically insulating portions by means of which said endportions are respectively sealed to and spaced from said centralportion; a pair of spaced electrodes housed within the enclosure forwhich said end portions serve as respective terminals, said electrodesextending towards one another through said insulating portion of theenclosure so that the gap between the electrodes lies within the centralportion of the enclosure; and a pair of sleeves of electricallyinsulating material carried on said electrodes respectively. each sleeveextending from a point near the inner end of the associated electrode toa point within the insulating portion of the enclosure through which theassociated electrode extends.

2. An arrester according to claim 1 wherein the outer end of each saidsleeve lies adjacent a section of the associated electrode which is asliding fit in the associated insulating portion of the enclosure.

3. An arrester according to claim 1 wherein each said sleeve iseffective to increase the breakdown voltage between any point on thesurface of the electrode carrying that sleeve and a conductive coatingwhich may be produced in operation on the internal surface of theadjacent insulating portion of the enclosure to a value above the dc.ignition voltage across the gap between the inner end of that electrodeand the adjacent part of the central portion of the enclosure.

4. an arrester according to claim 3 wherein each said sleeve projectsinwardly beyond the inner end of the adjacent insulating portion of theenclosure by an amount sufficient to make the shortest gap, via thegasfilling, between said end and the electrode carrying that sleeveappreciably larger than the gap between the inner end of that electrodeand the adjacent part of the central portion of the enclosure.

5. An arrester according to claim 4 wherein the por tions of theelectrodes projecting inwardly beyond the sleeves are of larger diameterthan the portions of the electrodes within said sleeves.

1. An excess voltage arrester comprising: a hollow cylindrical gasfilled enclosure comprising a tubular electrically conductive centralportion, a pair of electrically conductive end portions, and a pair oftubular electrically insulating portions by means of which said endportions are respectively sealed to and spaced from said centralportion; a pair of spaced electrodes housed within the enclosure forwhich said end portions serve as respective terminals, said electrodesextending towards one another through said insulating portion of theenclosure so that the gap between the electrodes lies within the centralportion of the enclosure; and a pair of sleeves of electricallyinsulating material carried on said electrodes respectively, each sleeveextending from a point near the inner end of the associated electrode toa point within the insulating portion of the enclosure through which theassociated electrode extends.
 2. An arrester according to claim 1wherein the outer end of each said sleeve lies adjacent a section of theassociated electrode which is a sliding fit in the associated insulatingportion of the enclosure.
 3. An arrester according to claim 1 whereineach said sleeve is effective to increase the breakdown voltage betweenany point on the surface of the electrode carrying that sleeve and aconductive coating which may be produced in operation on the internalsurface of the adjacent insulating portion of the enclosure to a valueabove the d.c. ignition voltage across the gap between the inner end ofthat electrode and the adjacent part of the central portion of theenclosure.
 4. an arrester according to claim 3 wherein each said sleeveprojects inwardly beyond the inner end of the adjacent insulatingportion of the enclosure by an amount sufficient to make the shortestgap, via the gas-filling, between said end and the electrode carryingthat sleeve appreciably larger than the gap between the inner end ofthat electrode and the adjacent part of the central portion of theenclosure.
 5. An arrester according to claim 4 wherein the portions ofthe electrodes projecting inwardly beyond the sleeves are of largerdiameter than the portions of the electrodes within said sleeves.